Disclaimer
This report was compiled by an ADRC visiting researcher (VR) from ADRC
member countries.
The views expressed in the report do not necessarily reflect the views ofthe ADRC. The boundaries and names shown and the designations usedon the maps in the report also do not imply official endorsement or
acceptance by the ADRC.

WIND INDUCED DAMAGE TO ROOFS
AC
Comparativep
StudySy on Roofs in Bhutan and JJapanp

Yeshey Lotay
Visitingg Researcher
Asian Disaster Reduction Center (ADRC)
April 22, 2015

Overview

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Background
Problem Statements
Scope and Objectives
Comparison of Japan and Bhutan Roofs
Recent Windstorm Damages
gAnalysis and Mitigations
Conclusions
Future Recommendations

Background• Bhutanese rural home are constructed by the carpenters
based on their indigenous knowledge.
• Rural home in Bhutan is non-engineered
non engineered construction•Till
Till early 1960’s
1960 s when Bhutan
is not open the other world,
peoplepp are using
g the wooden
shingle or other local
materials as roofing materials
which is more eco-friendly
and wind resistant

Changing period…..
• Roofing materials started changing

from are wooden shingles andcorrugated galvanized Iron (CGI)
sheets in early 1980s

WHY THEY CHANGE?
• Durability and affordability forthe rural people• Environment conservation•Less maintenance cost

Hip-and-gable roof with CGI sheets

Typical Bhutanese Rural Roof Configuration• RoofR f slopel
12⁰ – 15⁰
• Space (attic) between
roof floor and roof
covering is use for hay
and red ppepperppdryingy gand use as open-air
storage.

Attic(source: Traditional Bhutanese Houses survey and research report,2010)

Problem Statement• More than 60% of population lives in rural area in Bhutan
but people lack understanding on rural home roofs failure
during windstorm in Bhutan.
• In 2011 & 2013 windstorms, 2424 & 1017 rural home roofs
were damaged respectively• LackL k off windi d resistanti t t ffeatures
tincorporationiti dduringi
RECONSTRUCTION

2013 Windstorm damage

Reconstruction

Scope and Objective• To assess the rural home roofs in Bhutan based on
the roofs; configuration, shape, slope, overhang,
support string, connections and roofing materials
and comparing with the Japanese traditional roof• To assess the rural home roofs failure based ontheh existingi i rooff configuration
fii during
d i theh windstorm
i dbased on the literature review.
• Aim of this research is to help the common people
off Bhutan
Bh t tot reduce
dth rurall homes
thehrooff damage,deconomy losses during windstorm and to preserve
our traditionalt ditil BhutaneseBh tarchitectural
hit t l bybsuggesting some mitigations.

Comparison of Japan and Bhutan Roofs• Common rural home roofs in Bhutan:• Gable roof (Kirizuma)• Hip–and-gable roof(Irimoya)• Jamthok roof
• Lean-to roof
Lean-to and drangim roof
groof
• Drangrim

Gable roof

Hip-and-gable combined

Jamthok roof

Comparison of Japan and Bhutan Roof
• Common rural home roofs in Japan:• Gable roof (Kirizuma)• Hip–and-gable roof(Irimoya)• Hip roof (Yosemune)

Gable roof

Hip-and-gable roof

Hip roof

Comparison of Japan and Bhutan RoofSlNo.

Descriptionsi i1
Type of roof

BhutanJapangable, gable + hip roof (combined), lean gable(kirizum), hip(yosemune),roof, Jamthok (monitor) roof & etc.
gable + hip (irimoya), hogyo roof
and etc.
corrugated galvanized iron (CGI) sheet tiles, thatch, plank, shingles, bark &metal, shingles, thatch, banana leaves,etc..
bamboo, slates & etc.

2Type of roofmaterial

3

Roof
configuration

roof members act independently and norigid connections between members.

members are connected rigidly using
the screws or bolt or nails

4

Roof overhang

large roof overhang (500mm-2500mm)

less roof overhang (around 0 –1500mm)) usuallyy around 600mm

5Roof angle

roof slope between 12 °- 15°6Trusses
specificationsquare or rectangular shape tie beam,timber batten purlin, circular rafters

7

Roof-to-wallconnection

simply supported

roof slope various based on thelocation, e.g. in the snowy areas, roof
slope is steep
most members are circular timber intraditional roof and batten in modernhouses
rigid connections using bolt or nails

Recent Natural Disasters in Bhutan•Comparedpto other natural
disasters WINDSTORM eventsare more frequent in Bhutan

2011 & 2013 Windstorm Damages
Rural Home Roofs are
Vulnerable to strong wind

•In 2011 windstorm damage, 2424 rural home roofs
damaged out of 2598 total damaged structures (93.30%)•InI 2013 windstorm
i d t
ddamage, 1017 rurall hhome roofs
fdamaged out of 1093 total damaged structures (93.05%)

Wind Effect on Roof System

Positive and suction(negative) pressure
flow pattern

Net pressure
coefficientffi i tCommon Type of Roof Failure - 1

CGI sheet lift/loss at roof edges and gable roof end- Failure in timber section- CGI sheets are intact on batten- Timber batten section are notadequate to withstand windpressure

Common Type of Roof Failure - 2

Failure at roof ridge connections

TToe-nailsil•Timber lap joint withsingle 4-5inch toenail connection atroof ridge•PoorPoor connection

Common Type of Roof Failure - 3

Full blown-off
blown off or overturning of roof failure- No connection
roof-to-wallconnection- No connection
between struts
and attic floor

Analysis of Rural Home RoofGABLE ROOF SUFFERED
MAXIMUM DAMAGE

Site location

Gable Roof

Hip-and-gable RoofJamthok Roof

Analysis of Rural Home RoofSIMPLY SUPPORTED ROOF SUFFEREDMAXIMUM FULL/HALF DAMAGE

Half blown-off

Fully blown-off

Partly blown-off

Analysis of Rural Home RoofLARGER THE ROOF OVERHANG MORE
VULNERABLE DURING WINDSTROM

Overhang (2.1-2.5m)

Overhang (0.5 – 1.5m)

MITIGATIONSRoof Type Selections(Source: Roof type effect on uplift pressure (Bhandari et al., 2005))

HIP ROOF PERFORM BETTER THAN OTHER ROOF

MITIGATIONSRope Connection like Japanese roofs

Connect the rafters and purlins using rope/GI wires

MITIGATIONS

Use of metal strap and strong support string

MITIGATIONS

Use of sand bag (10-20kgs)
Use of stone over the CGI sheetMITIGATIONS•Use of gable end stiffener and overhang ties
•Use of additional purlin and bracing

MITIGATIONS•Use of sufficient side laps and end laps
•Use of rubber washer instead of bitumen washer

Conclusions• Bhutanese roof is highly vulnerable to strong wind because
most of the Bhutan rural home roofs were gable roof typewith large roof overhang between 500mm and 2500mm. It
was found that ggable roof is weak during
g the windstorm
especially with the large roof overhang.
• Bhutanese roofs act independently and roof configurations isvery flexible and weak during windstorm• Japanese roofs are wind resisted as compared to Bhutaneseroofs

Conclusions• Most common rural home roofs failure during the recent
windstorms are gable end failure,
failure ridge failure
failure, roof-to-wallconnection, timber section and pull through failure.
• Western part of Bhutan rural home roofs with large roof
overhang are more vulnerable to strong wind as compared tosouthern region roof with smaller overhang.

Recommendations•In this study, Bhutan rural home roof was assessed basedon the literature review and damage reports of 2011 and2013 windstorms. But it is not sufficient to conclude thatg theBhutan rural home roofs are still unsafe duringwindstorm. Therefore, in the future study, moreexperimental test is required to conclude the behavior of
Bh t rurall h
Bhutanhome roofs.
f•More study need to carry out on the roof joinery andconnection details.

Lets Help Community!
To Reduce the Damage from Windstorm